I. Field of the Invention
This invention relates generally to apparatus for measuring the pulse repetition rate of an electronic circuit, and more specifically to an interval-to-rate converter circuit suited for use in developing, measuring and displaying the repetition rate of the output from a signal generator.
II. Discussion of the Prior Art
To properly evaluate the performance of certain electronic equipment, such as a cardiac pacer, it is desirable to be able to measure the frequency or pulse repetition rate of that device and provide a visual display of that parameter in units of pulses per unit of time. Well known digital counting techniques are available for measuring the time interval between successive pulses. Typically, the leading edge of a first pulse in a sequence may open a gate which allows regularly occurring clock pulses to be entered into a counter. The gate is subsequently closed by the leading edge of the next pulse generator output pulse. The count thus developed in the counter is a measure of the interval between pulses. This interval is expressed in units of time, e.g., seconds or milliseconds. While the interval so measured is directly related to the pulse repetition rate, it is oftentimes desirable to present to a human user a rate reading in terms of pulses per unit of time.
Three different solutions to this problem have been proposed in the prior art. First, and perhaps most trivial, a printed table may be presented which correlates the interval measurement in units of time to a rate value in terms of pulses per unit of time. Typical of this conversion table approach is the Medtronic 5300 tester which includes a printed chart on the back of the device. One using this arrangement must take the interval measurement and then manually look up in the chart or table the rate value corresponding to this interval. This scheme suffers from many drawbacks, not the least of which is the fact that there is not sufficient room on the equipment to list all of the possible intervals and their corresponding rate values. Because the desired interval may not be present, it requires an interpolation with a resultant loss in accuracy. Furthermore, this approach is inconvenient and time consuming.
A second solution for developing a conversion between pulse-to-pulse interval and pulse rate requires the use of a calculator chip which typically requires at least two well regulated power supplies.
Still another possible approach involves the use of a read only memory (ROM) type look-up table. This latter approach suffers from two main drawbacks. First of all, it requires at least ten custom ROM's to obtain sufficient storage which would allow conversion of 3,000 different intervals to their respective rates. This necessarily increases the size of the test instrument and, in addition, is somewhat costly because of the custom nature of the ROM device.